When we examine the animal world in a descending series, from the highest animals to the lowest, we see how their organs of nutrition are gradually simplified, how they become less definite and less specialised in their functions, until we find that functions which are performed in higher animals by special glands are accomplished at the lower stages of the series by mere cells scattered in the tissues, or even by the whole protoplasm of the body. The same gradual simplification is seen in the organs of the senses. They also become less and less definite as we descend the scale; it becomes more and more difficult to separate them from each other, and in the lower invertebrates mere cells, disseminated in the tissues, answer more or less to the irritations from without. At last, at the very bottom of the series, the sense-irritations are received by the whole surface of the animalcule's body.

An immense amount of investigation has been made, especially within the last thirty years, in order to trace the chain of evolution of the sense-organs in the animal world, and to follow the gradual ascent of sense-impressions, from the mere irritability of protoplasm to the highly developed sensations of the higher animals. Anatomists, physiologists, and psycho-physiologists have joined in that colossal work, and by this time it may be said that a result of the highest importance for science altogether, and especially for psychology, has been attained. The series has gradually been reconstituted in full, through the efforts of scores of separate workers. The leading results of these wonderful investigations having lately been summed up by Dr. Wilibald Nagel in a suggestive introductory chapter to a more special work; 14 we may take it, together with a few other works, as a guide for a brief review of the subject.15

14 Dr. Wilibald Nagel, 'Vergleichend-physiologische und anatomische Untersuchungen über den Geruchs- und Geschmackssinn und ihre Organe; mit einleitenden Betrachtungen aus der allgemeinen vergleichenden Sinnesphysiologie,' in Leuckart and Chun's Bibliotheca Zoologica, Heft 18, I. and II. (Stuttgart, 1894 and 1895). A full bibliography will be found at the end of this work.

15 E. Jourdan's Les Sens chez les Animaux Inférieurs (Paris, J. Baillière, 1889) is an excellent little work on the same subject which can be safely recommended to the general reader. Unhappily it has not been translated into English. Haeckel's Essay

on the Origin and Development of Sense-Organs (English translation) dates from 1879. Romanes's Mental Evolution in Animals, Sir John Lubbock's The Senses and the Mental Life of Animals, especially of Insects, and C. Lloyd Morgan's Introduction to Comparative Psychology, published in 1894, although they do not exactly cover the same ground, are too well known to need recommendation. W. Wundt's Grundzüge der physiologischen Psychologie (4th edition, Jena, 1894), and Max Verworn's Allgemeine Physiologie are, of course, two classical works, rich in information upon this subject as well, but neither has yet been translated into English. Wundt's Lectures on Human and Animal Psychology have at last been translated into English in 1894. Some works on the lower organisms are indicated further on.

What most strikes a beginner in the study of the lowest animals is the variety of those of their acts and motions which apparently imply psychical life and consciousness. Those microscopical animalcules which consist of one single cell, or even of a mere speck of protoplasm, have evidently no traces of a nervous system; and yet their movements and their responsivity to external stimuli are such that one hesitates to interpret them as mere mechanical or chemical processes, such as we see in foams, or even as mere manifestations of 'irritability,' which is a property of all living matter.

In one of the American psychological laboratories, the daily life of a one-celled infusorium-a vorticella-was lately observed under the microscope for days in succession, and all the accidents of its uneventful existence were recorded.16 A transparent, tulip-like, or bell-like expansion at the end of a thin transparent stalk, which contracts at the slightest jerk; a tiny opening at the top of the bell, representing a sort of mouth, or rather a buccal pore; and a row of extremely fine cilia, which differ from hairs by being mere expansions of the protoplasm of the body-the whole, cilia and all, being covered with an extremely fine cuticle-such is that tiny infusorium which everyone possessed of a small microscope can find in a drop of water taken from a pond. Observed hour after hour under the microscope, while a feeble current of water was flowing over the glass slide, it was seen to swallow still smaller animalcules, after having attracted them into its 'mouth' by the motion of its cilia; it assimilated them, and being well provided with food, it reproduced itself by budding tiny vorticellæ from its sides.

To many stimuli it was insensible. Icy water was made to flow; bright light, immediately following darkness, was flashed upon the little creature; light of various colours was tried, as also musical sounds of all qualities and volumes'-the animalcule took no heed of them. But the slightest jerk or jar made it instantly contract its stalk; and it sorted with the greatest apparent precision the floating minute particles, swallowing those of them which suited it. The world of relation,' as psychologists say, of a vorticella thus consists of a series of touches, with perhaps some taste and smell impressions, hardly distinguished from each other. With all that, the vorticella displayed memory. When no other food was supplied to it but cells of yeast in sterilised water, it took first to the new food. It filled its body to distension with yeast cells; but in a few minutes the entire meal was suddenly rejected, and for several hours the vorticella could not be induced to repeat the experiment. It must have retained for several hours some unpleasant impression; it manifested powers of choice' as it ceased to swallow an unsuitable food; and in some

16 C. F. Hodge and H. A. Atkins, 'The Daily Life of a Protozoan,' in American Journal of Psychology, 1894-95, vol. vi. p. 524.

unknown way it discerned between yeast cells and the animalcules it was used to prey upon.

The vorticella is, however, a considerably developed being in comparison with, say, an amoeba, which consists of a mere speck of protoplasm, or with that slimy vegetable organism—a plasmodium— which is made up of thickly interwoven threads of naked protoplasm. And yet, even in these two lowest representatives of the animal and the vegetable world, something in advance of mere irritability appears. The amoeba avoids bright light, and if a pencil of light falls upon its path, it retreats; certain chemical substances attract it, while others act repulsively upon it; and when the two poles of a galvanic current are plunged into the drop of water the amœba lives in, it moves towards the negative pole and avoids the positive. As to the plasmodium, it displays a still higher discriminative power. For instance, two beakers, filled with water, are placed close to each other, and in one of them the water is kept at a temperature of 45° Fahrenheit, while in the other it is much warmer (86°). A strip of paper, upon which one of these myxomycetes fungi has spread itself, is then placed in such a way that one end of the paper dips into the cold water, while the other end touches the warm water. Immediately, the slimy fungus begins to slowly stretch out and draw in its threads, and after a time it will have crept over into the warmer beaker. Other plasmodia show their dislike of light by withdrawing from the lighted part of a surface into its shadowy part; while to chemical stimuli they are even still more sensitive. If a plasmodium be placed in a glass tube filled with boiled water, which contains no nutritious substances, and the tube be overturned and plunged by its open end into unboiled water, the fungus will creep out of the tube and spread itself in the water below. It also will invade a paper pellet saturated with the substance it usually feeds upon; but if a crystal of salt is placed on the paper which it is spreading upon, the fungus will at once withdraw its threads and shrink away from the unpleasant matter; and if, while it is spreading itself one way, its front end be cauterised with some acid, the whole plasmodium will at once change the direction of its motion. In short, these lowest organisms have the property of recoiling from harmful substances and of finding the useful ones all through the medium they are placed in. The lifeprocesses which are going on in their protoplasm, as its chemical composition is continually altered and reconstituted, are sufficient to result in a sort of discriminative power between what aids the process of life and what is liable to check it.17

"The literature of this subject is very large. It is, however, very well summed up in several works quite accessible to the general reader, namely:-Oscar Hertwig's Die Zelle und die Gewebe, Jena, 1893 (English translation published at Chicago in 1893); and Max Verworn's Psycho-physiologische Protistenstudien, Jena, 1889, which two contain full indexes of the original memoirs; also Binet's Psychic Life of Microorganisms, translated from the French, Chicago, 1890. In Wundt's Grundzüge, 4th edition, p. 25 sq., the psychological bearings of these researches are discussed.

With bacteria, the same phenomena become much more complicated. Bacteria, as a rule, are very sensitive to changes in the intensity of light-some of them preferring light and some others darkness as well as to electrical and chemical stimuli. When a diatom (one of the one-celled plants which swarm in fresh water, and look so pretty under the microscope, on account of their double symmetry and the bright green chlorophyll grains they contain) stops swimming about, bacteria will gather in thousands round it, and stand motionless, absorbing the oxygen it gives up. Suddenly the diatom will get out of the crowd of bacteria, but the crowd, after having remained in the lurch for a second or two, will soon follow it and reassemble again. And not only oxygen, but various chemical substances attract them as well. Going higher up the scale, when we come next to those infusoria which are provided either with a couple of thin threads (flagellæ), or are adorned with a row of fine cilia, we find them capable of performing co-ordinate movements which exclude all possible comparison with the purely mechanical movements taking place in simple foams. Thus, a flagellate infusorium will anchor itself by one of its flagellæ to a tiny fibre of weed, and continually work with the other flagellum in search for food; then it suddenly will jerk to the opposite side of its anchoring weed and continue there the same exploration. Or, while hunting, it will suddenly change the direction of its swim; or it will hunt by jerks. As to the sensibility of the lowest organisms to chemical stimuli, it is simply striking. They have their likes and dislikes 18 for different substances, and as they seek for some of them and avoid the others, they show an admirable discernment. The most wonderful fact, however, is that these microscopic beings can be attracted not only by substances which are necessary for their life, but also by some of those which are no food for them, or even are decidedly harmful (salicylic natrium, chloride of mercury, or morphine), but seem to please their tastes. Infusoria will thus abandon a medium containing nourishing substances, such as extract of meat, which they generally like, in order to intoxicate themselves with morphine. Drunkenness thus begins in the animal world at its lowest microscopical stages.

In the presence of a mass of such facts, and of the far more astounding powers displayed by all the lowest organisms in connection with their reproduction, shall we maintain that all these manifestations of life are physical processes, which have nothing in common with what we describe as psychical life at the higher stages? Or shall we not rather admit that what is described as 'irritability of living matter' at the lowest end of the series merges by imperceptible

18 The chemical sense of the lower organisms was admirably explored by Pfeffer, 'Chemotactische Bewegungen von Bacterien, Flagellaten und Volvocineen,' in Untersuchungen aus dem botanischen Institut zu Tübingen. Bd. iv. 1888; also his previous work in same periodical, Bd. i. 1884.

degrees into what we are bound to describe as 'sensibility'? In fact, it is impossible not to recognise that in the protozoans the first appearance, the very dawn of sensibility is met with; but, as shown by Verworn, that sort of sensibility belongs not only to the animalcule as a whole, but to each minute part of its body as well. When one of the cilia of a ciliated infusorium is irritated, the whole row is set into a wave-like motion in the proper direction, and the irritation spreads, not as in a corn-field, by each bent cilium pushing the next; it is transmitted through the underlying protoplasm, because the transmission of the irritation may be prevented by making an extremely fine transverse cut between two cilia. But if one cilium is cut out, together with a tiny portion of the protoplasm behind it— this almost incredible operation has been performed by Verworn-the isolated cilium answers the irritation in the very same way as if it continued to make part of the row in the animalcule's body. Each minute particle of the protozoan's body has thus the capacity of responding to the irritation; and the co-ordinated movements of the animal are a sum total of the movements of the particles. The protozoan, as Verworn says, can thus be compared to a crowd, in which there is no conception of the crowd's individuality, because each individual, on receiving a certain impression, acts for himselfthe final result being, nevertheless, a movement of the crowd.

It may, of course, be said that as the protozoans have no nervous system, they can have no psychical life. This is, at least, the opinion of Du Bois Reymond. But the nervous system, and even its nervecells, must also have had their embryonal stage in the evolution of the animal kingdom, and in the seemingly uniform protoplasm of the infusorium, which performs, together with other functions, some of the functions of the nerve-cells, we must have already the germs of the nerve-cell.19 In some infusoria there is even a tiny spot which seems to be more sensitive to light than the remainder of the body. However, it is needless to go high up the scale in order to find visible rudiments of a nervous system. They exist in the shape of nervecells and nerve-filaments in the next division, in which corals, medusæ, sea-anemones, and so on, are included (the colenterata). In this extremely interesting division,20 sensibility to light and sound, a low sensibility of the skin to pressure, and a high development of the chemical sense are found, together with a first specialisation of the sense-organs, and doubtless signs of voluntary action. In some subdivisions of this class (the comb-bearers, or ctenophores, and the sea-anemones) Nagel found the sense of taste highly developed ; while the movements which a ctenophore performs to prevent an 19 Max Verworn's Protistenstudien, p. 201.

20 W. Nagel has made it the subject of new detailed studies (Der Geschmackssinn der Actinien,' in Zoologischer Anzeiger, 1892, No. 400; 'Versuche über die Sinnesphysiologie von Beroë ovata und Carnarina hastata,' in Pflüger's Archiv, 1893, Bd. liv. p. 165).